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2.4.4 SpO2
The SpO2 value is obtained through the pulse waves of the finger tips based on specific
algorithm and clinical data. The SpO2 probe is the measurement transducer. It has two inbuilt
LEDs and an inbuilt light receiver. The two LEDs include one red-light diode and one infrared
diode, which emit light in turns. When the capillaries in the finger tip are iteratively congested
with blood pumped by the heart, the light emitted by the LEDs, after absorbed by the capillaries
and tissue, casts on the light receiver, which can sense, in the form of electric signal, the light
strength changing with the pulsated blood. The DC/AC ratio of the two photoelectric signals
corresponds to the content of the oxygen in the blood. Therefore, the correct pulse oxygen
saturation can be obtained with specific algorithm. Moreover, the pulse rate can be obtained
according to the pulse waveform.
The circuit of the SpO2 module is involved in four parts: SpO2 probe, signal processing unit,
LED-driven sequencing control part and the MCU.
2.4.5 TEMP
Temperature measurement principle:
1. The transducer converts the body temperature to the electric signal;
2. The amplifier amplifies the electric signal;
3. The CPU processes the data.
The circuit is a proportional amplifier consisting of operation amplifiers. When the temperature
reaches the heat-sensitive probe, the heat-sensitive probe generates the voltage signal, which is
sent to the A/D converter after being amplified. The probe detecting circuit is a voltage
comparator consisting of operation amplifiers. When the probe is disconnected, the voltage input
is lower than the comparing voltage, so the voltage comparator outputs the low level; when the
probe is connected, the voltage input is higher than the comparing voltage, so the voltage
comparator outputs the high level.
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